This invention relates generally to systems for attaching rigid or semi-rigid structures to human skin, and in particular to medical devices for attachment to the skin of the human body.
There are certain types of ambulatory medical devices such as miniature infusion pumps, iontophoretic devices and the like that are designed to be attached to human skin during use. Due to the nature of these devices many are rigid and attachment to the skin for a prolonged period of time has been unsuccessful. It is difficult to successfully adhere a rigid device to the skin surface for a prolonged period of time due to the rigidity of the device, the geightened center of gravity above the adhesion level, and the flexibility and pliability of the skin.
Rigid devices of the type disclosed in WO 97/10012 are presently applied to the skin by means of pressure sensitive adhesive. These devices show signs of failure within 24 hours of attachment and have completely detached before 48 hours have elapsed from the time of application. Such devices will be collectively referred to herein as rigid devices, except where otherwise specified.
Commercially available bandages or skin adhering tape employ the same types of pressure sensitive adhesive and reliably adhere to the skin for long periods of time, for example for one or more week(s). Conventional pressure sensitive adhesives consist generally of silicone, butyl or acrylic components that are formulated in such a way so that reliably adhere to the skin.
Bandages or skin adhering tape and present attachment means for rigid devices to the skin is that the adhesive system extends the adhesive to the edge of the device which is adhered to the skin.
It is at the interface between the rigid device and the skin that challenges adhesion, particularly in the case of rigid devices, and causes system failure.
The detachment of rigid devices from the skin typically occurs when there is a failure in the underlying skin integrity leading to separation of the device along with the outer skin layers. Reactions to external stress are responsible for most device to skin adhesion failures. In this regard, the areas of skin of greatest concern are the outermost layers of the epidermis, more particularly the stratum corneum. In this region of the skin there are multiple cell layers (as many as 150) which are held together or bonded by nodes known as desmosomes resulting in a flexible three-dimensional web. Although usually viewed as dead skin cells, the stratum corneum is, in fact a dynamic functional portion of the epidermis that provides protection for the lower skin layers. The structure of the stratum corneum is very compliant and has the ability to easily relieve stress applied thereto, thereby avoiding disruption of lower layers and maintaining the integrity thereof. When the skin is stressed, deep skin fracture is prevented because of the rupture of desmosome bonds through fatigue resulting in relief of stress through sloughing of surface layers of skin cells.
Skin is almost unmatched in its compliance and ability to relieve stresses. In particular, skin is about forty times more compliant than any commercially available pressure sensitive adhesive. Even the most compliant vinyl carrier for an adhesive will be about one hundred and thirty times less compliant than skin. A moulded plastics part of the type represented by a rigid device does not provide any compliance and would be close to zero on a relative compliance scale. Anything attached to the stratum corneum applies a stress to the skin. Thus, because of the difference in compliance between a rigid device and the skin there will always be a shear force between the skin and an adhesive system used to adhere said device to the skin. As indicated above, the skin relieves any stress by shearing.
Presently, rigid medical devices are completely attached along the lower surface to the skin by means of adhesive. The resultant rigid attachment maximises the stress potential on the skin leading to early signs of detachment and often complete detachment prior to the completion of the treatment provided by the device.
In addition, medical devices that infuse drug into a patient typically require the drug to travel from a container distal to the point of infusion. This results in a drug temperature change or flux that may be harmful to the efficacy of the drug and patient.
Accordingly, there is a need for a skin adhesive system that will adhere a rigid device to the skin for extended periods of time for use in effective therapy and diagnosis, as the case may be. Any such device should provide for both secure attachment and easy removal of the device. Additionally, effective adhesive systems for use in connection with infusion systems minimize the temperature flux of the drug which may optimize efficacy and overall patient benefit.
The present invention provides a system adapted to be disposed between a rigid or semi-rigid device and human skin for reliably attaching the device to the skin for an extended period of time, the system having a skin-contacting surface for adhering to the skin and an opposed surface, for attachment to the device over a portion of the total area of the adjacent surface of the device thereby minimising stress on the skin and reducing the tendency of the device to detach from the skin.
The system transmits any external stress to which the device is subjected and distributes it to the unattached areas of skin proximate to the adjacent surface of the device.
The system according to the invention provides for secure skin attachment for extended periods of time and easy removal of the device, when required. The system provides for superior performance relative to known systems and in tests has shown suitability for application to multiple sites and three-day operation or longer. In particular, the system according to the invention has been shown to be effective for more than 72 hours of secure device attachment to the skin while allowing the wearer to undergo the normal activities of daily life.
The system according to the invention by virtue of its construction provides sufficient relaxation of any stress before transition thereof to the skin. It will be appreciated that the lower the magnitude of any stress imparted to the skin, the better.
In one embodiment, the system comprises a structure which is a laminar element.
Preferably, the system extends beyond the adjacent surface of the device.
An important aspect of the present invention is that it minimises the forces on the interface between the device and the skin at the edge of the adhesive. This area must be the most compliant area of the system.
Thus, preferably each area attached to the skin is located towards the centre of the system so as to allow for flexibility towards the periphery thereof.
Further, preferably, the skin contacting surface comprises a pressure sensitive adhesive.
Still further, preferably, the system comprises a carrier element having viscoelastic properties approaching those of skin.
The carrier must be made of a compliant material or otherwise be capable of even distribution of stress, more especially a material that minimises local stress on the skin when a stress is applied to the device.
In a preferred embodiment, the system is attached to the adjacent surface of the device at a number of discrete areas.
Typically, three discrete areas or anchor points will be used.
Suitably, the areas of attachment to the device are arranged at the apices of a triangular area or other regular pattern or shape.
Thus, the structure is attached to the device over a limited area leaving the rest of said structure to move freely with the skin. This creates a flexible xe2x80x98skirtxe2x80x99 around each discrete attached area. Maximisation of the flexible area in the system according to the invention is a key component in the adhesion longevity achieved.
Extended wear time with the system according to the invention is accomplished by minimising the stresses imparted to the skin that cause local skin fatigue failure and eventual detachment of the device. The construction embodied in the system according to the present invention allows the skin to move more freely than in the case of prior art systems, thereby reducing the magnitude of any stresses on the skin.
The system can be permanently attached to the device at each area of attachment.
Alternatively, the system is detachable from the device.
This feature allows the device to be replaced without detaching the skin-contacting surface from the skin.
The system according to the invention can be used with any product that has a rigid or semi-rigid construction and has to be attached to the human body. Such devices include: infusion systems of the type covered by International Publication No. WO 97/10012; iontophoretic drug delivery systems; minimally invasive sensors, including glucose sensors; diagnostic devices such as devices used in heart rate, pulse and ECG monitoring; ostomy products; nerve stimulators; external programming, data collection and monitoring devices for pace makers and defibrillators; implantable hearing aids and the like.
In one embodiment, the system is attached to the device by means of the co-operating elements of a fastening system located on the base of the device and the opposed face of the system, respectively.
Adhesive attachment of the system to the device can be achieved by means of a locally applied adhesive such as a pressure sensitive, epoxy or heat or UV-activated adhesive.
In a further embodiment, the device is provided with a relief cavity which can accommodate deflection of the skin in use which would otherwise result in higher stress and a detachment thereof.
Thus, the base of the device can be contoured to allow a maximum flexibility in relation to the skin. For example, the base of the device can be designed with a concave or relieved area. This creates an area for skin to flex into during physical activity without it pressing against the device and thus increasing stress on the skin.
The relief cavity can be a concave area in the base of said device.
The device to be applied to the skin may be provided with an integral needle for delivery of a substance through the skin.
With such devices, a flexible area is preferably provided at the locus where the needle penetrates the structure for access to the skin.
The area where the needle penetrates the skin is also an adhesive edge and adhesion longevity is improved by creating a flexible skirt around the hole resulting from or allowing for the needle penetration. This flexible skirt would typically extend 1-2 mm radially from the adhesive edge. This skirt allows for stress relief when the needle moves in relation to the skin.
By providing a flat section around the area where the needle penetrates the system concentrates pressure against the skin during application. This allows maximum wetting of the skin surface by the adhesive to ensure effective device operation.
Also suitably the system is provided with a 2-3 mm diameter hole through which the needle penetrates.
A 2-3 mm diameter hole in the skin adhesive allows the device needle to the inserted into the skin without having to pass through the adhesive layer. The small size of the hole is necessary so that the skin remains taut to ensure penetration thereof by the needle.
Preferably, one area of attachment is provided with an extension to the periphery of the system which facilitates removal of the device when required.
In a further aspect of the invention there is provided a method for reliably attaching a rigid or semi-rigid device to human skin for extended period of time, comprising the steps of:
providing a system having a skin-contacting surface for adhering to human skin and an opposed surface for attachment to a rigid or semi-rigid device;
attaching the opposed surface to a portion of the total area of the adjacent surface of the rigid or semi-rigid device; and
attaching the skin-contacting surface to the skin, whereby when the device is subject to external stress, thereby minimising stress on the skin and reducing the chance that the device will detach from the skin.
Other objects, features and advantages of the present invention will become apparent upon reading the following detailed description of the embodiments of the invention, when taken in conjunction with the drawings and appended claims.